In motorcycles, a driving force is typically transmitted from a transmission of an engine to a rear wheel through a chain. In contrast, in shaft drive type motorcycles, the driving force is transmitted through a drive shaft extending in a longitudinal direction of a vehicle body instead of a chain (see e.g., Japanese Laid-Open Patent Application Publication No. Hei. 9-109981 or German Laid-Open Patent Application Publication No. 10232892). The shaft drive type motorcycles have the advantages that there is no need to adjust a stretch of the chain because of long-time use, or to frequently feed oil to the chain, and thus driving efficiency increases.
FIG. 9 is an enlarged view showing main parts of a drive shaft mounted in a conventional shaft drive type motorcycle. As shown in FIG. 9, a drive shaft 1 includes a first shaft member 2 having, at a front end thereof, a first bevel gear 2a which meshes with an output gear of a transmission of an engine. The first shaft member 2 is coupled to a second shaft member 4 via a first universal joint 3. The second shaft member 4 is coupled to a third shaft member 6 via a spline coupling portion 8 and a second universal joint 5. The spline coupling portion 8 has an engagement pin 10 at a spline on the second shaft member 4 side which is subjected to a radially outward force applied by a leaf spring 9 and is fitted into and engaged with an engagement hole 11 formed on a spline on the second universal joint 5 side. In a state where a second bevel gear 7 at a rear end of the third shaft member 6 is in mesh with a bevel gear mounted on a rear wheel not shown, the driving force is transmitted to the rear wheel.
The second shaft member 4 is inserted into a tubular drive shaft case 12 and is thus protected. A front end portion of the drive shaft case 12 is coupled to a transmission case 13 via a first rubber bellows-like tube 14, and a rear end portion thereof is coupled to a rear gear case 16 of the rear wheel via a second rubber bellows-like tube 15. The drive shaft case 12 has a small-diameter portion 12a with a diameter smaller than that of the universal joints 3 and 5. So, the second shaft member 4 is divided at the spline coupling portion 8 between the first universal joint 3 and the second universal joint 5 so that the second shaft member 4 is insertable into the drive shaft case 12 from forward.
However, since the drive shaft case 12 shown in FIG. 9 is a tubular unitary component, an operator needs to insert the drive shaft 1 into the drive shaft case 12 from an opening at one end of the drive shaft case 12, through an inside thereof, to an opening at an opposite end thereof. That is, the operator must move the drive shaft 1 over a large distance and therefore, operation is not easy. In addition, in order to insert the drive shaft 1 into the drive shaft case 12, it is necessary to provide the spline coupling portion 8 between the front universal joint 3 and the rear universal joint 5 to divide the drive shaft 1, increasing components of and assembly steps for the drive shaft 1. In order to couple the spline coupling portion 8 in an assembly process, it is necessary to make the second shaft member 4 and the second universal joint 5 to be coaxial with each other and to align the engagement pin 10 with the engagement hole 11. However, since the spline coupling portion 8 is covered with the case 12 and the bellows-like tube 15 and thus is invisible, the operator must remove the bellows-like tube 15 and see an inside of the bellows-like tube 15 to couple the universal joint 5 and the second shaft member 4. Therefore, the operation is not easy.